Many people with stoves in their homes or a firepit in their garden wonder whether ash for compost is any good, especially with wood ash.
When we talk about whether or not ashes are good for the garden, first of all it is important to look at what type of ash we are talking about.
Wood ash is good for the garden in certain specific circumstances and can be used in compost – in moderation. But coal ash, coke ash and other forms of ash from ‘smokeless fuels’ should never be used.
These other ashes are most often harmful for people, and for the plants we try to grow. And contain trace elements and heavy metals that should be kept away from food-producing areas and may be a danger to the environment. So use ash from untreated wood only if you want to add ash for compost.
If you wish to know more information about compost, why not check out our insightful article about how to reuse compost in your garden.
Table of Contents
What is Ash and Its Composition?
Wood ash is what is left over when wood is burned in a stove, fireplace or firepit or in industrial contexts. It is a powdery substance mostly made up of calcium compounds with some other trace elements present in the wood.
Both the wood burned (its type and age) and the rate of burning can influence the precise makeup of wood ash, but it typically contains carbon, calcium, potassium, magnesium, manganese, phosphorus and sodium, along with a range of trace elements in smaller quantities.
Calcium carbonate in wood ash accounts for one of its primary characteristics – its alkaline pH level. The alkalinity of wood ash is one of the main defining characteristics of this material and one of the main things that defines where it is useful and where it is not useful in a garden.
Benefits of Adding Ash for Compost
Wood ash is useful for adding to compost for a number of reasons. However, it must only ever be added little bits at a time, and never to excess. And compost must be covered to obtain the benefits that adding ash to the composting system can confer.
For one thing, adding wood ash to a composting system can prevent a mix from becoming too acidic.
This may be helpful in, for example, vermicomposting systems where overly acidic food waste may otherwise create conditions that are less than ideal for composting worms. But of course, it is important not to add too much wood ash or things could go too far the other way.
Wood ash can also be beneficial in compost in small amounts because it can bring its mineral content to the mix, and this includes a number of key nutrients and micronutrients that plants need to grow.
However, in an uncovered heap, the nutrients, which are water soluble, can easily leach away and be lost from the finished compost. So wood ash may be more usefully added to systems where the compost is covered.
A study that tested composts with 10 and 20% wood ash added to other composting materials and processed in a drum composter found that adding ash increases oxygen levels which is good for composting bacteria and other microorganisms. They also found that wood ash increased the temperature of the compost.
This can be a good thing, since it can speed up decomposition (as in hot composting systems) and create a finished compost more quickly. But temperatures that get too high can reduce microbial diversity which is not always what we want.
Ash was indeed shown in the study to increase the speed of the composting process and also reduced nitrogen loss through that process over a two year period.
A different study found that plants grew better when soil was amended with an ash compost than they did with a compost not containing ash. One of the composts tested had 8% ash and another 16%.
Concerns and Precautions
As mentioned above, though ash can be very useful in moderation, using it in compost is not always the best idea, especially when it is used injudiciously.
Potential for Over-Alkalinity
One of the primary concerns with using wood ash in compost is of course that the extremely alkaline wood ash can make your compost too alkaline overall.
A good compost should have a balanced or slightly acidic pH to deliver what plants need, whether you are using the compost you create in pots and containers, or spreading it on your garden beds or borders.
So using too much wood ash in the mix can alter the pH, and throw the compost off balance, changing the composition of the mix and impacting all the life found within an aerobic compost.
Presence of Heavy Metals and Harmful Chemicals
Ash can contain heavy metals and if treated wood was used, it may contain other harmful chemicals too. The metals content in most untreated wood ash is considered to be at acceptable levels for organic gardening.
But it is still a good idea to limit how much ash you add to a compost system to be on the safe side, especially if you will use the finished compost around food.
Safe Storage and Application
Of course it is important to consider safety as you take wood ash from the source and transport it to your composting system. Make sure that it is not hot before you use it, and take care not to breathe in the fine powder as you take it into your garden and add it to your composting system.
Best Practices for Using Ash in Compost
By now, it should be clear that adding ash to a composting system can be beneficial, but that you do need to be circumspect and moderate in doing so. Here are some tips for best practice to make sure you get it right.
Sourcing and Storage
First things first, remember that you should only use untreated wood ash in your composting system. Other types of ash from fossil fuels are not suitable at all.
If you have a small firepit or chiminea outside, you may be able to transfer ash (as long as you don’t use it too frequently and generate too much) direct to your composting system.
If you have a wood burning stove, it is likely that you will generate more ash than you can use in your composting system. Fortunately, there are many other ways to use it. Store it carefully in a dry, safe container and use just small amounts from this container in your composting system.
There are different schools of thought on how much ash you can add to your composting system. As mentioned above, ash is not a good ingredient for all forms of composting.
And the amount you should add will also depend in part on what composting method you choose, as well as the other ingredients in the mix.
A reasonable rule of thumb however is to add approximately 1 part by volume of ash to 10 parts of other compostable materials and to mix the ash in well so that it is dispersed more evenly through the mix.
With a mix of this kind you should see the benefits without experiencing the potential disadvantages of adding ash to a composting system.
Remember, for a good quality compost you need to add a wide range of different compostable materials alongside the wood ash.
There needs to be a good mix between the ‘green’ or nitrogen rich materials like green leaves, grass clippings, vegetable scraps, etc. and the ‘brown’ or carbon rich materials like cardboard, untreated paper, dried leaves, wood chips or other woody material, or straw.
What precisely you should add to the mix, and in what precise proportion will depend on the specific composting method that you choose.
Alternatives to Using Ash in Compost
Since the primary benefits of using ash for compost is the nutrients it can add to the mix, it can be useful to look at other sources that can provide some of the same nutrients that might be added to composting systems where adding ash is not necessarily the best idea.
For example, rather than adding ash for calcium, you might consider adding crushed eggshells, which are also rich in calcium.
To provide the potassium and other nutrients that ash can provide within a compost, you might use a range of leafy material from dynamic accumulator plants. Dynamic accumulator plants are plants particularly effective at gathering and storing certain nutrients and matter from these plants is richer than most plant sources in those particular nutrients.
Charcoal is another form of ash that you could use for your garden, but first, you need to consider the benefits and hazards that this form of ash for compost can bring to your garden.
Using Ash Compost in a Polytunnel
If you do create a good quality compost with ash in the mix, then this will be a valuable soil amender for the soil in a polytunnel garden.
It can be especially useful as a soil amendment where a garden has acidic soil, where it can help to reduce acidity. And could be a problem if the soil is already alkaline. But I most cases, where the ash was not overused in the compost, can still be useful for adding nutrients to the soil.
Some gardeners also recommend using fungal compost in their gardens. Find out why now.
Can ash compost be used for all types of plants?
While many plants benefit from the minerals in ash compost, some plants prefer acidic conditions and might not thrive with too much ash. It’s essential to know the pH preferences of your plants before adding ash compost.
How often should I add ash to my compost?
It’s recommended to sprinkle a thin layer of ash into the compost occasionally, rather than adding large amounts at once. Regularly test your soil’s pH to ensure it remains balanced.
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Kuba, T., Tschöll, A., Partl, C., Meyer, K. & Insam, H., (2008). Wood ash admixture to organic wastes improves compost and its performance. Agriculture, Ecosystems & Environment, 127(1–2), pp.43-49. [online] Available at: https://www.sciencedirect.com/science/article/abs/pii/S0167880908000650?via%3Dihub [accessed 28/08/23]
Hughes, T., (2023) The Benefits and Hazards of Using Charcoal Ashes in the Garden. Lahinch Taven and Grill. [online] Available at: https://lahinchtavernandgrill.com/the-benefits-and-hazards-of-using-charcoal-ashes-in-the-garden/ [accessed 28/08/23]
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.
To get in touch, visit https://ewspconsultancy.com.